[0001] The present invention relates to a method for reading data from a data page for optical
data storage in an optical storage system, e.g. a holographic storage system, including
the correction of missing or wrong positioned sync marks, and to an apparatus for
reading from an optical storage medium performing this method.
[0002] The invention is described below using a holographic storage system as an example.
It is apparent to a person skilled in the art that the invention is applicable within
other optical storage systems.
[0003] In holographic data storage digital data are stored by encoding the interference
pattern produced by the superposition of two coherent laser beams, where one beam,
the so-called 'object-beam', is modulated by a spatial light modulator (SLM) and carries
the information to be recorded. The second beam serves as a reference beam. The interference
pattern leads to modifications of specific properties of the storage material, which
depend on the local intensity of the interference pattern. Reading of a recorded hologram
is performed by illuminating the hologram with the reference beam using the same conditions
as during recoding. This results in the reconstruction of the recorded object beam.
[0004] One advantage of holographic data storage is an increased data capacity. Contrary
to conventional optical storage media, the volume of the holographic storage medium
is used for storing information, not just a single or few two-dimensional layers.
One further advantage of holographic data storage is the possibility to store multiple
data in the same volume, e.g. by changing the angle between the two beams or by using
shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored
as data pages. Typically a data page consists of a matrix of light intensity variations,
i.e. a two-dimensional binary array or an array of grey values, which code multiple
bits. Data pages consisting of patterns showing different phases can also be used.
This allows achieving increased data rates in addition to the increased storage density.
The data page is imprinted onto the object beam by the SLM and detected with a detector
array.
[0005] Data pages include synchronisation marks, also referred to as sync marks, to determine
the exact scaling factor from the SLM to the detector and to correct image distortion.
Sync marks usually consist of a specific bit pattern, which is known and can be identified
clearly by the reading apparatus. For any holographic data storage system the correct
sync mark detection is essential for a successful demodulation procedure. As the scaling
factor and the image distortion can vary locally, sync marks are usually distributed
over the entire data page. If the sync mark detection fails in a part of the data
page then in most cases the demodulation will also fail in this region. Due to defects
in the holographic material or distortions such as detector noise the correct detection
of a local sync mark may fail.
[0006] It is an object of the invention to propose a method for detecting such failures
and/or correcting wrong positioned or missing sync marks. Readout is improved by using
estimated sync marks.
[0007] According to the invention, the method for reading data from a data page from an
optical data storage medium has the steps of:
- identifying at least one missing or wrong positioned sync mark on a read data page,
- estimating a corrected sync mark position, and
- using the estimated corrected sync mark position for further processing.
[0008] According to this method, wrong positioned or missing sync marks can be identified
even if a number of sync mark detections failed. Nevertheless, it is assumed that
most of the sync marks are detected correctly to provide a basis for the calculation
of the estimated corrected sync mark position. Using the proposed sync mark correction
allows to determine image distortion and locally varying scaling factors on a data
page reliably. The method is numerically simple and efficient. It can be implemented
independent from the underlying modulation scheme of a data page. After the replacement
the possibility to demodulate the data correctly in the region of the formerly wrong
positioned sync mark increases. The bit error rate is reduced.
[0009] Advantageously, the method is used for reading from a holographic data storage medium.
In a holographic data storage system, data are stored using two-dimensional data pages.
In this case, it is of particular importance to determine image distortion and locally
varying scaling factors previous to demodulation. To perform this determination reliably,
correct sync mark positions are needed. Increasing the accuracy of the sync mark detection
increases the reliability of the readout process.
[0010] Advantageously, further data processing according to the invention includes data
detection in the region of the missing or wrong positioned sync mark using the estimated
sync mark position. Data is read out reliably essentially independent from wrong positioned
or missing sync marks.
[0011] Favourably, wrong positioned or missing sync marks are corrected by estimating their
correct position by interpolating sync marks from corresponding rows and/or columns.
Sync mark positions are estimated, e.g. by the intersection of two regression curves
of the corresponding row and column. This leads to an accurate estimation of the corrected
sync mark position.
[0012] Advantageously, the corrected sync mark position is estimated by interpolating sync
marks from a set of sync marks nearby. Within a defined area around a wrong positioned
or missing sync mark, other sync marks are used to determine the correct position
of the wrong positioned or missing sync mark. Consequently, local differences of the
optical path and of the optical elements are considered during estimation of the corrected
sync mark position.
[0013] Favourably, a measure for the deviation of a sync mark position from its expected
position is calculated to detect wrong positioned or missing sync marks. The measure
is calculated, for example, using regression curves of the corresponding row or column,
or the measure is calculated using other sync marks within a defined area around an
estimated sync mark position. The mathematical measure is calculated, for example,
using quadratic filtering of the read out sync marks or using the statistical variance
of the sync mark positions. The mathematical measure can be adapted to an algorithm
which fits best to the system configuration. The invention is flexible and adjustable
to the basic conditions.
[0014] Favourably, a data area around a wrong positioned sync mark is shifted according
to a sync mark deviation. This improves read out accuracy.
[0015] According to a further aspect of the invention an apparatus for reading a data storage
media uses a method according to the invention for detecting missing or wrong positioned
sync marks and correcting the sync mark positions.
[0016] Favourably, in a holographic storage medium the sync marks have a light intensity
distribution and a spatial frequency distribution similar to the light intensity and
spatial frequency distribution of the data blocks. A similar light intensity and spatial
frequency distribution over the whole data page leads to a more uniform utilization
of the holographic material resulting in a higher storage capacity. Using the method
for sync mark detection according to the invention, the light intensity of the sync
marks can be reduced and adapted to the light intensity distribution of the whole
data page. The sync mark detection errors resulting from the lower light intensity
of the sync marks are corrected by the invention.
[0017] For better understanding the invention shall now be explained in more detail in the
following description with reference to the figures. It is understood that the invention
is not limited to this exemplary embodiment and that specified features can also expediently
be combined and/or modified without departing from the scope of the present invention.
- Fig. 1
- schematically shows a data page including sync marks and data blocks,
- Fig. 2
- shows a data page, including four rows and four columns of sync marks,
- Fig. 3
- shows a part of a data page, including one wrong positioned sync mark,
- Fig. 4
- shows a part of a data page, including the corrected position of a sync mark,
- Fig. 5
- shows an example of the replacement of a wrong positioned or missing sync mark within
a data page.
- Fig. 6
- shows a read out data page that has sync marks with a light intensity similar to the
light intensity of the data page
- Fig. 7
- schematically shows an apparatus for reading and/or recording a holographic storage
medium.
[0018] Fig. 1 schematically depicts a part of a data page 1 including two rows 5 and three
columns 4 of sync marks 2. A sync mark 2 is composed of e.g. a 5x5 pixel pattern.
Different sync marks 2 can be used and distributed over the data page 1 to put additional
information into the sync marks 2. The sync mark detection is realized e.g. by searching
for local maxima in the correlation of the read out data page and the sync mark pattern.
The distance between two adjacent sync mark columns 4 is dx. The distance between
two adjacent sync mark rows 5 is dy. The offset from the upper left corner of the
data page 1 to the first sync mark position is ox and oy, respectively. Data blocks
3 are indicated schematically by dots.
[0019] Fig. 2 shows an example of a part of a data page 1 as a combination of digital patterns.
A pattern like this is imprinted on an SLM during writing. Sync marks 2 have a specific
shape and can be retrieved from the figure. This data page 1 contains four sync marks
2 in a row 5 and four sync marks in a column 4. In this example, sync marks 2 are
equally spaced. The positions of the sync marks 2 on a data page 1 and the pattern
of the sync marks 2 are also known in advance by the reading apparatus. The white
lines are for clarification only and divide the data page 1 into subpages, each one
containing 4x4 blocks. One of the blocks is a sync block 2.
[0020] Fig. 3 schematically shows a part of a data page 1, including sync marks 2 arranged
in rows 5 and columns 4. The dotted lines indicate a row 5 and a column 4 of sync
marks. They are not the same as the lines indicated in Fig. 2 showing the subpages.
There is one wrong positioned sync mark 6 indicated. The correct detection of the
sync mark failed. Sync marks in row 5 and column 4 are assumed to be detected correctly.
The expected position of sync mark 6 is at the intersection point of the interpolation
curves of sync marks in row 5 and column 4. The distance of a sync mark 6 from the
expected position for this sync mark can be expressed by a mathematical measure, e.g.
the variance. If this measure exceeds a certain limit, it is assumed that the sync
mark detection failed. In this case, the corrected position of the sync mark is estimated
to be at the intersection point of the interpolation curves of the correctly detected
sync marks. The sync mark which was identified to be detected at a wrong position
is shifted to this intersection point. Fig. 4 shows the replacement of the sync mark
7.
[0021] Fig. 5 shows an example of the detected data pattern depicted in Fig. 2. Sync marks
2 can be identified in the picture as bright areas. Generally, sync marks 2 have a
higher ratio between 'on' and 'off' pixels compared to a data block 3. Nevertheless,
on a detected data page, they cannot be identified by eye. The data page 1 shown in
Fig. 5 uses sync marks 2 with an even higher light intensity compared to the data
blocks 3 for demonstration purposes only.
[0022] The distance between two adjacent sync marks 2 is known in advance. Therefore, a
grid 9 can be established on a data page 1. Between four neighbouring grid points
9, a sync mark 2 has to be detected. In case a part of the data page 8 is not detected
correctly, also the sync mark detection within this area fails. This can be caused
e.g. due to a bad signal-to-noise ratio (SNR), local defects in the holographic material
or local defects of the detector or the optical apparatus. Without using the method
according to the invention, in the area 11 between four neighbouring grid points 9
the sync mark 7 is not readable. The sync mark is determined at a data block 6 which
looks most similar to a sync pattern. All four subpages 10 using the erroneous sync
mark 6 as a boundary show significant read errors. In contrast, using a corrected
sync mark 7 according to the invention allows reading all data within the four surrounding
subpages 10 except for the data in the defect region 8. A significantly reduced error
rate arises.
[0023] Fig. 6 shows a data page 1 using sync marks 2 with a light intensity and a spatial
frequency distribution similar to the light intensity and spatial frequency distribution
of the whole data page 1. The sync marks 2 can not be identified by eye. The SNR on
the sync mark 2 is worse compared to the SNR of the sync marks 2 shown in Fig. 5.
[0024] In Fig. 7 an apparatus 20 for reading and/or recording a holographic storage medium
29 is shown schematically. A source of coherent light, e.g. a laser diode 21, emits
a light beam 22, which is collimated by a collimating lens 23. The light beam 22 is
then divided into two separate light beams 26, 27. In the example the division of
the light beam 22 is achieved using a first beam splitter 24. However, it is likewise
possible to use other optical components for this purpose. A spatial light modulator
(SLM) 25 modulates one of the two beams, the so called "object beam" 26, to imprint
a two-dimensional data pattern. Both the object beam 26 and the further beam, the
so called "reference beam" 27, are focused into a holographic storage medium 29, e.g.
a holographic disk or card, by an objective lens 28. At the intersection of the object
beam 26 and the reference beam 27 an interference pattern appears, which is recorded
in a photo-sensitive layer of the holographic storage medium 29.
[0025] The stored data are retrieved from the holographic storage medium 29 by illuminating
a recorded hologram with the reference beam 27 only. The reference beam 27 is diffracted
by the hologram structure and produces a copy of the original object beam 26, the
reconstructed object beam 30. This reconstructed object beam 30 is collimated by the
objective lens 28 and directed onto a two-dimensional array detector 32, e.g. a CCD-array,
by a second beam splitter 31. The array detector 32 allows to reconstruct the recorded
data.
1. Method for reading data from a data page from an optical data storage medium,
having the steps of:
- identifying at least one missing or wrong positioned sync mark on a read data page;
- estimating a corrected sync mark position; and
- using the estimated corrected sync mark position for further processing.
2. Method according to claim 1, wherein the method is used for reading from a holographic data storage medium.
3. Method according to claim 1 or 2, wherein the further data processing includes data
detection in the region of the missing or wrong positioned sync mark using the estimated
sync mark position.
4. Method according to one of the preceding claims, wherein the corrected sync mark position is estimated by interpolating sync marks from corresponding
rows and/or columns.
5. Method according to one of the preceding claims, wherein the corrected sync mark position is estimated by interpolating sync marks from a
set of sync marks nearby.
6. Method according to one of the preceding claims, wherein a measure for the deviation of a sync mark position from its expected position is
calculated..
7. Method according to one of the preceding claims, wherein a data area around a wrong positioned sync mark is shifted according to a sync mark
deviation.
8. Apparatus for reading from an optical data storage media, characterized in that it is adapted to perform a method according to one of claims 1 to 7.